1. Field of the Invention
This invention relates to an additive for a rubber composition and an additive composition for a rubber composition capable of improving a processability and productivity in the production of rubber articles without badly affecting properties of an uncured rubber and of a cured rubber as well as a rubber composition and a pneumatic tire using the same.
2. Description of Related Art
In the production of rubber articles such as tires, belts, hoses and the like, kneading of a rubber composition is generally conducted in order for uniformly dispersing compounding ingredients and filler into starting rubber. And also, it is required to further conduct rubber milling for plasticizing the rubber composition several times in order to facilitate building operability at subsequent rubber processing step. However, such a repetition of the plasticizing operation is unfavorable in view of the productivity of the rubber article. On the other hand, although it is effective to decrease the rubber milling number in order to improve the productivity of the rubber article, the uncured rubber composition is not sufficiently plasticized when the rubber milling number is merely decreased, which does not ultimately link to the improvement of the productivity.
To this end, it is possible to improve the processability of an uncured rubber composition by adding a plasticizer, a processing aid or the like. Then, the building operability can be improved while decreasing the rubber milling number. In case of using a conventional plasticizer, a processing aid and the like, however, the degradation of properties of the uncured rubber composition or the cured rubber composition is caused and it is actually difficult to decrease the rubber milling number. At the processing step in the production of the rubber article including natural rubber is particularly existent a polymer gel formed by entanglement of natural rubber molecular chains with each other, or by reaction of functional groups in isoprene chain of natural rubber with each other, or by reaction of such a functional group with non-rubber ingredient in natural rubber, so that the rubber milling number increases and the degradation of the building operability is accompanied therewith. And also, if the rubber milling number is increased for plasticizing an uncured rubber composition, the lowering of molecular weight in natural rubber is caused, which badly affects the properties of the cured rubber composition.
Therefore, it is demanded to improve the building operability without degrading the properties of the uncured rubber composition or the cured rubber composition in the production of the rubber article particularly including natural rubber since early times.
In order to solve these problems, there are recently proposed some attempts. For instance, JP-A-11-209406 discloses that a quantity of polymer gel in natural rubber is decreased by adding an aromatic polycarboxylic acid derivative to natural rubber. According to this method, however, it has been confirmed that blooming is apt to be easily caused in the cured rubber.
And also, JP-A-6-57040 and JP-A-4-20579 disclose that the processability is improved by adding a specified ester. According to these methods, however, peculiar odor is generated in the processing, which becomes a problem in view of the operating environment.
It is, therefore, an object of the invention to solve the aforementioned problems of the conventional techniques and to provide an additive for a rubber composition and an additive composition for a rubber composition both being capable of improving the processability without degrading the properties of the rubber composition. It is another object of the invention to solve the problem in view of the operating environment and particularly decreasing the quantity of polymer gel in a rubber composition including natural rubber without lowering the molecular weight of natural rubber to thereby improve the processability and the productivity and stabilize surface states of both uncured and cured rubber compositions as well as a rubber composition and a pneumatic tire using the same.
The inventors have discovered that slippage between rubber molecules is increased by using a partial ester between a specified aliphatic or aromatic polyvalent carboxylic acid and a (poly)oxyalkylene derivative as an additive for a rubber composition without degrading the properties of a cured rubber composition and as a result the invention has been accomplished.
According to a first aspect of the invention, there is the provision of an additive for a rubber composition consisting essentially of an ester selected from an ester of (i) an aliphatic polyvalent carboxylic acid with (iii) a (poly)oxyalkylene derivative, and having at least one carboxyl group in its molecule, and an ester of (ii) an aromatic polyvalent carboxylic acid with (iii) a (poly)oxyalkylene derivative, and having at least one carboxyl group bonded to an aromatic ring in its molecule.
According to a second aspect of the invention, there is the provision of an additive composition for a rubber composition comprising (a) a reinforcing filler and (b) an ester selected from an ester of (i) an aliphatic polyvalent carboxylic acid with (iii) a (poly)oxyalkylene derivative, and having at least one carboxyl group in its molecule, and an ester of (ii) an aromatic polyvalent carboxylic acid with (iii) a (poly)oxyalkylene derivative, and having at least one carboxyl group bonded to an aromatic ring in its molecule, in which a compounding ratio by weight of the component (a) to the component (b) is 70/30 to 30/70.
According to a third aspect of the invention, there is the provision of a rubber composition comprising a rubber ingredient and an ester selected from an ester of (i) an aliphatic polyvalent carboxylic acid with (iii) a (poly)oxyalkylene derivative, and having at least one carboxyl group in its molecule, and an ester of (ii) an aromatic polyvalent carboxylic acid with (iii) a (poly)oxyalkylene derivative, and having at least one carboxyl group bonded to an aromatic ring in its molecule.
According to a fourth aspect of the invention, there is the provision of a rubber composition comprising (A) a rubber ingredient and (B) an additive composition for a rubber composition consisting of (a) a reinforcing filler and (b) an ester selected from an ester of an aliphatic polyvalent carboxylic acid with a (poly)oxyalkylene derivative, and having at least one carboxyl group in its molecule, and an ester of an aromatic polyvalent carboxylic acid with a (poly)oxyalkylene derivative, and having at least one carboxyl group bonded to an aromatic ring in its molecule.
According to a fifth aspect of the invention, there is the provision of a pneumatic tire characterized by using a rubber composition defined in any one of the third and fourth aspects of the invention.